This applications is based on application No. H11-086077 filed in Japan on Mar. 29, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a color integrating optical device, and more specifically, to a color integrating optical device for projecting different pieces of color information from a plurality of display panels after integrating the pieces of information.
2. Description of the Prior Art
Conventionally, in color integrating optical devices, a so-called multi-plate method is frequently used for image display devices used as display panels. This is particularly intended for performing color display. In order to perform color integration when color display is performed, conventionally, a dichroic mirror using surface reflection or a cross dichroic prism has been mainly used.
However, in the structure of the dichroic mirror using surface reflection, since reflection is directly performed at the dichroic film on the surface, some of the light rays from the image display devices pass through a glass plate serving as the substrate and the other of the light rays do not pass there through according to whether the light rays are reflected at the dichroic mirror or not, so that the lengths of the optical paths to the projection surface differ among the light rays. Consequently, the amounts of the caused astigmatic differences are different, which makes correction difficult. As a result, the image quality is degraded.
To make the lengths of the optical paths the same, recently, a dichroic mirror comprising glass plates cemented together has been used, for example, as described in Japanese Laid-open Patent Application No. H5-107501. The dichroic mirror of this prior art comprises two glass plates cemented together. The two glass plates have the same thickness, and dichroic coating is applied to one of the opposing surfaces thereof. In such a dichroic mirror, since both the transmitted light rays and the reflected light rays pass through the two glass plates, these light rays are optically absolutely equivalent. As the dichroic mirror, one having the above-described structure is mainly used.
Concrete examples of a conventional color integrating optical device using a dichroic mirror includes one successively performing color integration with two dichroic mirrors, for example, as described in Japanese Laid-open Patent Application No. H4-175742. Another example is one performing color integration by use of a dichroic mirror where dichroic coating is applied to the cemented surfaces of the glass plates cemented together as described above in order to correct the difference of the optical path caused because of the dichroic mirror as described in Japanese Laid-open Patent Application No. H4-310913.
Another example is one in which although a dichroic mirror using surface reflection is used, the lens back focal length is long and a dummy glass plate is disposed in order to correct the difference of the optical path caused because of the dichroic mirror. Specifically, one has been proposed in which the lens back focal length is long enough for three plates inclined with respect to the optical axis to be disposed between the projection optical system and the image display devices as described in Japanese Laid-open Patent Application No. H4-216543, or one has been proposed in which the lens back focal length is long enough for four plates inclined with respect to the optical axis to be disposed between the projection optical system and the display =panels as described in Japanese Laid-open Utility Model Application No. H6-25831.
However, in the structure as described in Japanese Laid-open Patent Application No. H4-175742, since the number of dichroic mirrors through which the light rays from the image display devices pass differs among the image display devices, the lengths of the optical paths to the projection surface are different and the amounts of the caused astigmatic difference are different as mentioned above, so that when the number of pixels increases, the performance is unacceptable.
In the structure as described in Japanese Laid-open Patent Application No. H4-310913, although the lengths of the optical paths are the same, since the dichroic mirror has the structure in which light is reflected by the dichroic film provided on the rear surface of a glass plate or in the vicinity of the rear surface, so-called unnecessary light rays are generated.
FIGS. 5A and 5B schematically show how such unnecessary light rays are generated. In these figures, reference numerals 1 and 2 represent display panels, reference numerals 3 and 4 represent condenser lenses of the display panels 1 and 2, respectively, and reference numeral 5 represents a dichroic mirror. The dichroic mirror 5 has a dichroic film 6 formed on the rear surface viewed from the reflection side (the side of the display panels 2) in FIG. 5A and on the obverse surface viewed from the reflection side (the side of the display panel 2) in FIG. 5B.
Assume now that a light ray emanating from the display panel 1 and shown by the arrow a passes through the condenser lens 3 and is transmitted by the dichroic mirror 5, that a light ray emanating from the display panel 2 and shown by the arrow xcex2 passes through the condenser lens 4 and is reflected at the dichroic mirror 5 and that these light rays are integrated into a light ray shown by the arrow xcex3. At this time, although a reflection preventing film 7 is provided on the surface where the light ray of the arrow xcex2 is incident on the dichroic mirror 5 (the obverse surface) as shown in FIG. 5A, reflection is not completely prevented, and part of the light ray of the arrow xcex2 is reflected at the surface to become an unnecessary light ray as shown by the dashed arrow a.
Further, part of the light ray of the arrow xcex2 normally reflected at the dichroic film 6 is further reflected at the reflection preventing film and is again reflected at the dichroic film 6 to become an unnecessary light ray as shown by the dashed arrow b. These unnecessary light rays form an unnecessary image in the vicinity of the image formed by the normal light rays as shown by the arrow xcex3, so that the image performance deteriorates.
On the contrary, when the dichroic film 6 is provided on the surface where the light ray of the arrow xcex2 is incident on the dichroic mirror 5 (the obverse surface) as shown in FIG. 5B, since merely the light ray of the arrow xcex2 is reflected as it is, no unnecessary light ray is generated. Instead, in this case, since the light ray of the arrow xcex1 passes through the dichroic mirror 5 and the light ray of the arrow xcex2 does not pass therethrough, the lengths of the optical paths from the display panels 1 and 2 to a non-illustrated projection surface are different as heretofore mentioned, so that the amounts of the caused astigmatic differences are different to make correction difficult. As a result, the image quality is degraded.
In FIG. 5B, there are cases where the reflection preventing film 7 is provided on the surface where the light ray of the arrow xcex1 is incident on the dichroic mirror 5 (the rear surface), and even if reflection is not completely prevented and part of the light ray of the arrow xcex1 is reflected at this surface to become an unnecessary light ray as shown by the dashed arrow c, the unnecessary light ray does not become a problem because it never reaches the projection surface. In this case, the reflection preventing film 7 is used rather for increasing the transmittance to thereby improve the image efficiency.
In the structure as described in Japanese Laid-open Patent Application No. H4-216543 or Japanese Laid-open Utility Model Application No. H6-25831, the long lens back focal length causes a problem. That is, when liquid crystal display panels are used as the image display devices, since liquid crystal has a characteristic that the display properties thereof vary according to the angle of incidence of illumination light, to illuminate the liquid crystal display devices at the same angle in order to avoid the display property variation, it is necessary to illuminate the liquid crystal display panels with a telecentric optical system. In a telecentric optical system, the lens diameter of the projection optical system should be extremely large when the lens back focal length is long.
FIGS. 6A to 6C are views schematically explaining the problem involved in the illumination of the liquid crystal display panels. Mentioning the conclusion at first, by providing a condenser lens 102 before a display panel 101 so that the optical system is substantially telecentric in the part immediately before the display panel 101 as shown in FIG. 6A, a light ray L from the display panel 101 is incident on a projection optical system 103 without diverging even if the optical system is mainly non-telecentric and the lens back focal length is comparatively large, so that the lens diameter xcfx861 can be reduced.
On the contrary, when the lens back focal length is long in an optical system being telecentric as a whole, the light ray L from the display panel 101 diverges as shown in FIG. 6B, so that the lens diameter xcfx862 of a projection optical system 104 on which the diverging light ray L is incident is extremely large. This requires extra space for disposing the projection optical system 104 and increases the cost. The structure as described in Japanese Laid-open Patent Application No. H4-216543 or Japanese Laid-open Utility Model Application No. H6-25831 involves such a problem.
A lens back focal length being longer than necessary places an enormous load on the optical system, so that it is difficult to obtain excellent optical performance. When the optical system is non-telecentric as a whole as shown in FIG. 6C, even if the size of the projection optical system 103 can be reduced, the display panel 101 is not illuminated at the same angle because of the absence of a condenser lens, so that the angle shown by 6 differs, for example, between the central part and the peripheral part of the image plate as shown in the figure. This results in a nonuniform image. The light ray L in FIGS. 6A to 6C are illustrated in a form serving also as illumination light incident on the display panel 101 in the part immediately before the display panel 101.
In the firstly mentioned structure using a cross dichroic prism, although no problem arises in image quality and unnecessary light rays, a large glass block is necessary because the volume is generally large. In addition, since four right angle prisms are cemented together and the surfaces sandwiching the right angle sides are coated with a dichroic film, extremely high accuracy is required for cementing the prisms, so that the cost is extremely high.
An object of the present invention is to provide a color integrating optical device performing color integration using mirrors, being low in cost and realizing excellent image quality.
To achieve the above-mentioned object, according to the present invention, a color integrating optical device for integrating different color rays from a plurality of display devices each of which has a condenser lens in front thereof and emitting the integrated color rays to a non-telecentric projecting optical system, comprising:
a dichroic mirror plate having a dichroic film, the dichroic mirror integrating the color rays by the dichroic film in which a color rays from one of display devices reflects thereon and another color rays from the other of display devices transmits thereon, the dichroic film being formed an incident surface of a color rays to be reflected;
a dummy plate provided on an optical path of color rays to be reflected by the dichroic film so that an unnecessary rays caused by part of the color rays being reflected at the dummy plate does not emit to the non-telecentric projecting optical system; and
wherein the following condition is fulfilled:
2 less than {LBxe2x88x92(t÷cos xcex8)xc3x973xc3x97n}÷D less than 5
xe2x80x83where:
LB is a distance from a most integrating optical device side surface of the non-telecentric optical system to the condenser lens;
n is the number of plates disposed on the optical path;
xcex8 is a tilt angle of the thickest plate with respect to the optical axis of the non-telecentric optical system;
t is a thickness of the thickest plate; and
D is a length of the display device in direction parallel to a plate formed by the color ray are bent by the dichroic mirror plate.
Moreover, according to the present invention, a display optical system comprises:
A color integrating optical device for integrating different color rays from a plurality of display devices each of which has a condenser lens in front thereof and emitting the integrated color rays to a non-telecentric projecting optical system, comprising:
a dichroic mirror plate having a dichroic film, the dichroic mirror integrating the color rays by the dichroic film in which a color rays from one of display devices reflects thereon and another color rays from the other of display devices transmits thereon, the dichroic film being formed an incident surface of a color rays to be reflected;
a dummy plate provided on an optical path of color rays to be reflected by the dichroic film so that an unnecessary rays caused by part of the color rays being reflected at the dummy plate does not emit to the non-telecentric projecting optical system; and
where in the following condition is fulfilled:
2 less than {LBxe2x88x92(t÷cos xcex8)xc3x973xc3x97n}÷D less than 5
xe2x80x83where:
LB is a distance from a most integrating optical device side surface of the non-telecentric optical system to the condenser lens;
n is the number of plates disposed on the optical path;
xcex8 is a tilt angle of the thickest plate with respect to the optical axis of the non- telecentric optical system;
t is a thickness of the thickest plate; and
D is a length of the display device in direction parallel to a plate formed by the color ray are bent by the dichroic mirror plate; and
a projection optical system for projecting the image light rays,
wherein an optical member for correcting an astigmatic difference caused by the color integrating optical device is provided in the projection optical system.